This invention relates generally to incandescent lamps and, more particularly, to incandescent lamps having envelopes that carry infrared (IR)-reflective coatings. The invention also relates to lighting fixtures incorporating such lamps.
Incandescent lamps of this kind, having envelopes that carry IR-reflective coatings, typically in the form of multi-layer stacks of dielectric materials, are well known in the lighting industry. Such dielectric coatings include alternating layers of high-refractive index materials, e.g., niohia (Nb2O5), tantala (Ta2O5), and titania (TiO2), and low-refractive index materials, e.g., silica (SiO2), wherein the layer thicknesses are controlled to be substantially one quarter the wavelength of the light to be reflected by constructive interference. The successive layers of such coatings are typically created using physical vapor deposition (PVD), reactive sputtering, chemical vapor deposition (CVD), or plasma-impulse chemical vapor deposition (PICVD) to deposit various oxides onto a substrate, such as glass.
Multi-layer dielectric coatings can be designed to be highly reflective in a range of wavelengths and highly transmissive in other wavelengths. For example, a dielectric coating that reflects IR light, usually in the range of 750 to 1600 nanometers (nm), but that transmits other wavelengths of light, is commonly called a “hot mirror” or an “IR coating.” The transition from reflecting wavelengths to transmitting wavelengths can be made very narrow, typically about 50 nm or less.
IR coatings were first combined with quartz-halogen lamps in the late-1980s, to increase an incandescent lamp's luminous efficacy. Incandescent light sources typically produce about 10-15% visible light and about 85-90% IR light. An IR coating on an incandescent lamp's transparent envelope reflects a substantial portion of the IR light emitted by the lamp filament back onto the filament. The filament absorbs a portion of that IR light, thereby reducing the amount of electrical power required to heat the filament to a given temperature and consequently increasing the lamp's luminous efficacy. Lamps incorporating linear filaments have exhibited improved luminous efficacy as high as 40%. For example, an FCM linear lamp has a luminous efficacy of 28 LPW, while an IR-coated FCM/HIR linear lamp of equal luminous flux has a luminous efficacy of 39 LPW.
IR-coated quartz halogen lamps generally are available in two form factors: “linear lamps” and “elliptical lamps.” Linear lamps generally include a long, single-coiled filament and a concentric tubular envelope. Most of the IR light reflected by the coating is redirected back to the filament, because the filament is a cylindrical object concentric with the cylindrical IR-coated envelope. Elliptical lamps generally include a short, coiled-coil filament and an elliptical envelope. The IR-coated elliptical reflector is configured with its two foci located approximately at the ends of the filament. For this reason, most of the IR light reflected by the coating is redirected back to the filament, and large end losses associated with short filaments are avoided.
Transparent conductive coatings (TCCs), formed of materials such as indium tin oxide (ITO), have been widely used in products where it is desirable to make a non-conducting substrate, such as glass, electrically conductive yet highly transmissive to visible light. By appropriately varying the doping and thickness of the TCC and by controlling the deposition process, a coating can be made to have a visible light transmissivity greater than 85% and to be electrically conductive (e.g., about 20 Ω/square). Such a coating also has the property of having a reflectivity to IR light that increases gradually at longer wavelengths. In one example, a typical 200-nm thick ITO coating is about 8% reflective at 1000 nm, 45% reflective at 2000 mm, and 72% reflective at 3000 nm. The wavelength at which transmittance and reflectance of this coating are equal, also known as the “plasma frequency,” is approximately 1850 nm.
IR coatings used in the past with quartz-halogen lamps generally transmit on the order of 5 to 30% of IR light in a wavelength range of 740 to 1600 nm; 20 to 90% of IR light in a wavelength range of 1600 to 2200 nm; and greater than 75% of IR light at wavelengths above 2200 nm. Because dielectric coatings have very little absorption at these wavelengths, it follows that such prior art IR coatings reflect 70 to 95% of IR light in the range of 750 to 1600 nm; 10 to 80% of IR light in the range of 1600 to 2200 nm; and less than 25% of IR light above 2200 n. Peak IR emittance from a typical tungsten filament operating at 3000° K (color temperature) is known to occur at about 920 nm, and more than half of the IR power from such a filament is located in a wavelength range of 750 to 1600 nm. Consequently, prior art coating designs generally have been thought to be highly effective at redirecting most of the IR light back to the lamp filament.
Another prior art IR coating design, which is disclosed in U.S. Pat. No. 6,476,556 to E. Cottaar, includes an interference film having a transmittance that averages at least 90% in the visible wavelength range of 400 to 760 nm and having a reflectance that averages at least 75% in the infrared wavelength range of 800 to 2200 nm. Preferably, the interference film has a reflectance that averages at least 85% in the infrared wavelength range of 800 to 2500 nm.
In general, prior art IR coatings for quartz halogen lamps are designed to reflect the maximum integrated IR power generated by the light source. In other words, the coatings have been designed to maximize the integrated sum of reflection at each wavelength above 700 nm multiplied by the radiated power of the filament at the same wavelength. Designers of such prior art IR coatings also have sought to maintain maximum visible transmission, generally at values greater than about 90%.
The IR coating designs described briefly above have proven to be effective in improving the luminous efficacies of incandescent lamps. However, there remains a continuing need for an improved lamp, and for a lighting fixture incorporating such a lamp, exhibiting yet a higher luminous efficacy. The present invention fulfills this need and provides further related advantages.